Deletion-Restriction for Logarithmic Forms on Multiarrangements

Abstract

We consider the behaviour of logarithmic differential forms on arrangements and multiarrangements of hyperplanes under the operations of deletion and restriction, extending early work of Günter Ziegler. The restriction of logarithmic forms to a hyperplane may or may not be surjective, and we measure the failure of surjectivity in terms of commutative algebra of logarithmic forms and derivations. We find that the dual notion of restriction of logarithmic vector fields behaves similarly but inequivalently. A main result is that, if an arrangement is free, then any arrangement obtained by adding a hyperplane has the "dual strongly plus-one generated" property. One application is another proof of a main result of a paper by the first author characterizing when adding a hyperplane to a free arrangement remains free. A further application is to resolve two conjectures due to Ziegler, which we defer to a sequel.